Node.js Interview Questions 2026

Tip: Node uses a single-threaded event loop (libuv) with phases: timers, pending callbacks, poll, check (setImmediate), and close. Blocking work is offloaded to a thread pool. process.nextTick and Promise microtasks run between phases, before timers. Never block the loop with synchronous CPU work.

Tip: process.nextTick() callbacks run immediately after the current operation, before the event loop continues — they can starve I/O if overused. setImmediate() runs in the check phase, after the current poll phase completes. Use setImmediate for "run after I/O", nextTick sparingly for cleanup.

Tip: Offload to worker_threads (shared memory, good for CPU work), child_process for separate processes, or the cluster module to fork the server across CPU cores. For heavy compute, a job queue (BullMQ + Redis) processed by separate workers keeps the request path responsive.

Tip: Middleware are functions with (req, res, next) that run in order on the request pipeline — used for logging, body parsing, auth, and error handling. Calling next() passes control onward; error-handling middleware takes four args (err, req, res, next) and is registered last.

Tip: Streams process data in chunks instead of buffering it all in memory — Readable, Writable, Duplex, and Transform. Piping (readable.pipe(writable)) handles backpressure automatically. Use them for large files, HTTP bodies, and data transformation to keep memory flat.

Tip: Synchronous code uses try/catch. Callbacks use the error-first convention (cb(err, data)). Promises use .catch(); async/await uses try/catch around awaited calls. Unhandled promise rejections crash the process in modern Node — always attach a handler or wrap in try/catch.

Tip: require is synchronous, dynamic, and resolved at runtime (CommonJS). import is static, hoisted, supports tree-shaking, and can be asynchronous (ESM). ESM is enabled via "type":"module" or .mjs. You cannot freely mix them; interop requires createRequire or dynamic import().

Tip: Common causes: unbounded caches, uncleared timers, and growing global arrays or event-listener accumulation (watch for MaxListenersExceededWarning). Detect with heap snapshots in Chrome DevTools / clinic.js and by monitoring process.memoryUsage().heapUsed over time. Bound caches with an LRU and remove listeners on teardown.